The pleiotropic cytokine interleukin (IL)-15 has been heavily implicated in the pathogenesis of autoimmune diseases like lupus and rheumatoid arthritis because it is found at significantly elevated levels in afflicted patients, and because it is known to play critical roles in the proliferation and homing of lymphocytes. Thus, IL-15 is a potential pathogenic and therapeutic target in the rheumatic diseases; however, definitive experimental evidence remains lacking. We propose in this pilot and feasibility study to explore the mechanisms by which interleukin-15 might participate in the pathogenesis of systemic humoral autoimmunity using mutant animal models. Specifically, we will: 1. Determine the role of interleukin-15 in murine lupus, and 2. Investigate the role of interleukin-15 in the regulation of autoreactive B cell proliferation.